Recently, a surface emitting body has attracted attention which is used as a backlight for various kinds of displays, a display board such as a signboard or an emergency light or a light source for an illuminator or the like, since it has many advantages such as high luminous intensity, high efficiency, small thickness and light weight.
In the surface emitting body, an organic electroluminescence element (hereinafter also referred to as an organic EL element), employing an organic material and emitting light by application of electric energy across the positive and negative poles, can emit light by application of a low voltage of from several volts to several tens of volts. Furthermore, the organic electroluminescence element is particularly noted from the viewpoint that it is a thin and totally solid state element and therefore is advantageous in space saving.
The organic EL element is formed of a transparent substrate and provided thereon, a first electrode, a light emission layer and a second electrode in that order. Voltage being applied across the first electrode and the second electrode, holes injected in the light emission layer from the first electrode, an anode and electrons injected in the light emission layer from the second electrode, a cathode are recombined in the light emission layer, thereby emitting light. This emitted light transmits through the first electrode and the transparent, substrate and is observed by users as light emitted from the organic EL element. It is possible that the wavelength of light emitted from the light emission layer is changed by the kinds of dopants added to the light emission layer.
When a surface emitting body is used as a light source for an illuminator, it is ordinarily required that the light source emit white light. As a method for emitting white light by means of an organic EL element, there is a method which incorporates, in the light emission layer, different emission materials emitting a different light in the form of lamination layers or as a mixture thereof, thereby emitting and mixing lights of various colors to obtain a white colored light.
However, in a thin surface emitting element such as an organic EL element, when the output angle of light, determined by the refractive index of the thin light emission, layer and that of the medium through which the output light passes, is over the critical angle, the emitted light is totally reflected and enclosed within the element, and vanishes as a guided wave. As a result, emitted light except for light output from the front surface of the element vanishes, resulting in lowering of light extraction efficiency (being the ratio of the energy of light emerging outside the element to the energy of light emitted).
The light extraction efficiency (emission efficiency) in the front direction derived from multi-reflection based on classical optics can be approximated to ½ n2 and is almost determined by the refractive index n of the light emission layer. When the refractive index of the light emission layer of an organic EL element is about 1.7, the efficiency of emission from the emission portion of the organic EL element is about 20%. The rest of the emission travels in the direction of the light emission layer plane (dissipates in the transverse direction) or vanishes at the metal electrode on file side of the light emission layer opposite the transparent electrode (is absorbed on the rear side of fire element). That is, the organic EL element emits light at the inner portion of a layer (having a refractive index of 1.7 to about 2.1) with a refractive index higher than that of air, and only extracts 15 to about 20% of light emitted in tire light emission layer.
As a method for improving the light extraction efficiency, disclosed is a method in which a prism sheet or a micro lens array is arranged on the surface of a transparent substrate to form a concave-convex form such as a prism form or a lens form, thereby preventing the total reflection at the interface between the transparent substrate and atmospheric air (refer to for example, Patent Document 1). This method can provide high luminous intensity, but causes chromaticity change or luminous intensity variation with the angle of tire observation. Therefore, there is problem in applying the method to an organic EL element for a white light emission illuminator.
Hitherto, as a backlight of a liquid crystal display, a light diffusion sheer is known which comprises a transparent support and provided thereon, a light diffusion layer with a concavo-convex surface containing a binder resin and resin particles (refer to for example, Patent Documents 2 and 3), In this sheet, the concavo-convex layer containing the resin particles formed on the light output side of the sheet is provided for the purpose of reducing the luminous intensity unevenness due to the light scattering effect. Patent Document 2 discloses a sheet having on the reverse side thereof a concavo-convex layer containing resin particles, but the concavo-convex layer is formed in order to improve the slipping property, but has no influence on the light extraction. Further, a lens sheet is known, which contains fine particles at the lens supporting portion or within the lens (refer to for example, Patent Document 4). This sheet is one forming the lens structure on the light output side by resin molding, and can obtain an extraction effect of light exceeding the critical angle, however, light scatter due particles contained within the sheet is insufficient to suppress chromaticity change with the angle of observation fax the organic EL element. Thus, an attempt to employ a light diffusion sheet for backlight as a light extraction sheet for an organic EL element has been made, however, this light diffusion sheet is employed for the purpose of suppressing luminous intensity unevenness or improving the front luminous intensity in the backlight unit, but is insufficient to improve the performance of for organic EL element. Thus, a novel light extraction sheet for an organic EL element has been sought.
As a light extraction sheet for an organic EL element, disclosed is a film wife a concavo-convex surface which has a haze of not less than 70% and a total optical transmittance of not less than 80% (refer to Patent Document 5). The concavo-convex surface of the film disclosed in this Patent Document utilizes a alight concavo-convex form caused during drying and curing of the resin layer. Therefore, this concavo-convex surface is not sufficient in the light extraction effect, and further has no effect of suppressing color change with the angle of observation.